Contact urticaria.

Contact urticaria is an immediate wheal response generated by a wide variety of contactants in susceptible individuals. These contactants can be divided into three categories based on their mechanism of action: nonimmunologic, allergic, and unknown. The list of known illicitors of CU is growing rapidly, and the syndrome is being recognized more often. CU to certain preservatives in foods and pharmaceuticals may get increased attention in the future because of such widespread exposure to them both through contact and systemic absorption. A range of systemic reaction patterns can be seen in allergic CU, in some of the physical urticarias, and in exposure to ammonium persulfate. The danger of such systemic reactions should be borne in mind when performing rechallenge patch testing in the office.